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A thoroughly sporadic column from astronomer Mike Brown on space and science, planets and dwarf planets, the sun, the moon, the stars, and the joys and frustrations of search, discovery, and life. With a family in tow. Or towing. Or perhaps in mutual orbit.

Most people will probably think of tomorrow as the 5 year anniversary of the demotion of former-planet Pluto. That seems fair; the Pluto demotion got all of the news, caused all of the fights, and promoted all of the discussion. But now that tempers have cooled and the world has come to terms with a new more scientific eight-planet solar system, it is time to remember the other important thing that happened on that day five years ago. On August 24th 2006 the International Astronomical Union (IAU) defined a new class of objects in the solar system: the dwarf planets.

As you will recall, the IAU declared that planets are the objects which go around the sun and gravitationally dominate their orbits. In our solar system, the eight planets are unique in that behavior. But there are other much smaller bodies out there – Pluto being the most famous – that look like planets (simply meaning that they are round) but are not dominant. Pluto and many of these other objects all circle the sun in similar orbits in what is called the Kuiper belt. These objects are the dwarf planets.

At the time this new class of dwarf planets was proposed, the IAU also declared that three dwarf planets were then known: Ceres (the largest asteroid), Eris (the newly discovered largest Kuiper belt object which precipitated all of this mess), and Pluto. In the entire five years since then, the IAU has declared two other objects to be dwarf planets: Makemake and Haumea.

A reasonable person might think that this means that there are five known objects in the solar system which fit the IAU definition of dwarf planet, but this reasonable person would be nowhere close to correct. By my best estimate there are possibly 390 known dwarf planets in the solar system (don’t worry, I’ll explain below).

What is going on here?

This is a really really really bad representation of the solar system

The problem is with the IAU. When the IAU defined dwarf planet they also declared themselves gatekeepers of what counts and what doesn’t. While that might not sound like a big deal at first, it is actually quite a deviation from standard scientific practice. In every other part of astronomy, if I make a discovery – let’s say a new brown dwarf – I write a scientific paper announcing the new discovery to the world. Other scientists can then go observe the object and decide, perhaps, that I was mistaken. They then write a paper saying that, no, it wasn’t a brown dwarf, it was a distant galaxy, and the scientific debate continues. At no point is the IAU required to ever enter the object on the official brown dwarf or official distant galaxy list, because no such official lists exist.

Why are dwarf planets different? Why is there a gatekeeper at all? My suspicion is that the gatekeeping is a leftover from when there was a chance during all of the debating 5 years ago that dwarf planet would be declared a subclass of planet. Everyone feels the need to be able to answer the simple question: how many planets are there? If dwarf planets counted, too, someone needed to be keeping track. At least I suspect that was the logic. (I might note that it’s probably good that dwarf planets never got to be called planets simply because it would be embarrassing for the IAU to be wrong about the number of planets in the solar system by a factor of ten or more.)

Today, we don’t need to know the precise number of dwarf planets because school children will have to memorize them all, we need an estimate of the true number of dwarf planets so we can continue to explore and understand the outer solar system. For this new understanding to emerge, dwarf planets need to be rescued from the IAU. Declaring themselves gatekeepers was bad enough, but then they mostly just locked the gate and stopped looking at what was happening outside. As far as I know, there is no process by which the official dwarf planet list expands. The only two objects that were added – Haumea and Makemake -- were added somewhat haphazardly simply because they were also being named at the time. The IAU declared this new class of objects and then locked it up in a tower and let it die

Does any of this matter? I would argue that it does. Our definitions and our concepts are our ways that we explain the solar system to people. Saying that there are 8 planets is a powerful statement about the fact that there are a small number of bodies which dynamically dominate the workings of the solar system. Saying that there are already possibly 390 icy dwarf planets known beyond the planets showcases the excitement of the last decade of discovery and points to the new frontiers at the outer solar system. Saying, instead, that there are 5 dwarf planets paints the incorrect picture that these 5 objects are somehow a rare breed. They are not. They are all over. And they must be freed!

How do we free the dwarf planets? Through science. In honor of the fifth anniversary of the creation of dwarf planets, I have spent some time putting together what I think is currently the best list of possible dwarf planets out there. Doing so is not precise. We don’t know the sizes of many objects. We don’t know how reflective their surfaces are. We certainly don’t know their shapes. But science is OK with uncertainty. I have made the best estimates that I think can be made, and, more importantly, I have documented how I made the estimates, which means that people can examine them, disagree with them, and proposed why better estimates should be made. This process is very different from gatekeeping; this process is science.

You can read my list of possible dwarf planets here. It updates every day as new objects are discovered and more information about the sizes and shapes of objects in the Kuiper belt becomes known. Lists are just lists, though. The important part is the methodology, which you can read all about here. Not only can you read all about it, you can also reproduce I, change it, and disagree with it, because, again, that is the way with science.

According to my estimates, today we know of 390 objects which are possibly dwarf planets, of which 102 are probably dwarf planets, 60 are likely dwarf planets, 31 are highly likely dwarf planets, and 8 are nearly certainly dwarf planets. What do I mean by those subjective-sounding terms? Again, you can read my definitions here. And you can disagree with them.

And what about the official IAU list? What should we do with it? I believe it has done enough damage to people’s understanding of the outer solar system over the past five years. I suspect that even the IAU doesn’t really want to be in the gatekeeping business and they just accidentally painted themselves into that corner. They should abandon it.

The IAU does have one other good option, though. They could attempt their own analysis of which objects fit their definition. And they could publish the analysis. And others could comment and agree and disagree. What would we call it if that happened? I think the name, again, is science.

48 comments:

Told about this, the president of IAU Division III just mailed me that "I will discuss this with the IAU EC" (which is the Executive Committee, their highest body). Let's see if they change the ad hoc rule from 2008 that specifies a minimum absolute brightness for even considering a Kuiper Belt Object a dwarf planet which seems way too high now: an object with H = +1 is over 1200 km across; all of Mike's candidates for ascension are fainter than +2.

I always considered the dwarf planet category unofficial, with the only official reckoning being whether a body is registered in the Minor Planet Catalog. Are you proposing the roundish minor planets get a separate catalog all their own?

@anon No, I'm proposing the opposite. There currently **is** an official dwarf planet list with the IAU as the list keeper. I am simply proposing that we honestly deal with the question of "how many roundish minor planets are there in the solar system" by using science.

It does go back to the argument of whether roundness is a significant distinction among minor planets, Vesta and Ceres being the usual main belt examples of sufficient vs insufficient roundness. If partial melting of the interior, surface geology, or accompanying moons don't require the body be a relaxed spheroid, why does it matter?

Thanks, Dr. Brown, for sharing this information. I'm a 6th grade Science teacher in rural Pennsylvania, and I teach a unit on the Solar System. Before this embarking on this endeavor with my students each year, I check to see what the IAU has done since the previous year. I find all of this immensely fascinating, and the 11- and 12-year-olds in my classes usually do as well. Wow! Maybe 390 dwarf planets? They will be thrilled, until we start talking about creating mnemonic devices to remember all of them…

Thanks for taking the time to write this. Being not even amateur astronomists but having a deep love for all of this, me and my ten year old son know all about you and didn't know much about what you describe here.

My son will be excited to read all of this and ditto both of us to go to your links, especially that list of other candidates. You've done a good, good thing and helped fill my son (and I) with wonder and love of science. It is much appreciated. And with him, I am seeing it will go far (he has curiosity and a gift for mathematics).

I have been expecting the IAU to revisit this issue, since they left some gaping holes in their resolutions in 2006. My feeling is that the rules and classification system that is used to categorize the myriad objects in our solar system must be outlined and codified in such a way that it may be applied to the planet swarms we find in orbit around other stars. Roundness, or degree of "sphericity" and location relative to other planets just won't cut it as we learn more about those star systems over the coming years, decades and, yes, centuries.

Thanks for the post, i was wondering for a while why IAU did not announce more dwarf planets. Another question which is most likely irrelevant for professionals, but why did not more dwarfplanet candidates get names? ( I know the rules ..) still did no "discoverer" or "discovering team" come up with name suggestions? Or was that delayed till a possible confirmation of the status of the object? The numbers-letter-combinations are hard to memorize, even nicknames help a lot in telling the different KBOs apart.

Ayyyy references. I know I know. The more automated I can keep the whole thing the more likely it will stay updated. But when day when I am procrastinating and not getting any work done perhaps I'll add the references.....

@anonymous on names: Many of the larger candidates are ones the we discovered, and we have always been reluctant to name something until we know enough about it to give it an appropriate name (i.e. see Snow White this week!). The others I can't speak to; maybe their discoverers are likewise waiting!

Since I was one to help "demote" Pluto in Prague, even though I wrote some papers about it, I think that the IAU decision was logical and timely. There was a need to retain a small number of "planets" that kids could remember. The issue whether there is or is not an official list of dwarf planets is neither here nor there, as is the issue about naming them. Since there IS a rule, and it requires sphericity, let's stick to this and accept as dwarf planets all bodies that have definitely been shown to be spherical.

Are the IAU's rules for naming dwarf planets exactly the same as those for the smaller solar system bodies? If they're different, I could see why the IAU might want to maintain a separate list for the dwarfs.

Either way, it seems they've been way too slow in updating that list. Maybe they've been influenced by reading "The Hobbit". You know -- once you invite the dwarfs in, there's no stopping them....

What makes you think that the IAU considers itself to be a gatekeeper here? Or that others (including you) consider it to be a gatekeeper? I'm not being combative here, I really want to know why, since I've encountered it often enough. Is there a published, official list?

There is a list at http://planetarynames.wr.usgs.gov/Page/Planets, but it's clearly not intended to be exhaustive, since, e.g., it only lists 50 moons for Jupiter. It also makes no claim to be official or exhaustive (or current -- no P4).

The IAU hasn't declared any further dwarfs, but, if they don't consider themselves a gatekeeper, they wouldn't. They only have to stick their necks out when naming. Even there, they clearly state that their criterium (H<1) is for naming only, not strict categorisation.

Anyway, at 1.6 Yg, Quaoar is a dwarf planet whatever any list says. It might be elongated, too, like Haumea and Varuna. There's gotta be a good story behind having so many of those. (hint hint...)

I think the biggest problem with the IAU definition of "planet" (and there are many) is that it takes a term used for centuries by the general population and tries to redefine it into a precise scientific term. But the data are actually messier than the definition they wanted, so they had to "dumb down" the science, and ended up with a definition that doesn't serve any real purpose. And in the end, the only real criterion that people use to judge planets in their mind is size: "How BIG is it relative to Earth?" That's where the real definition lies. Everything else is just details. ;)

My full take on the definition of "planet": http://www.alienrobotzombies.com/2011/04/theyre-all-planets-part-1.html

Lastly, why should we care if there are more than eight planets? There are more than eight states in the U.S., more than eight countries in the world, more than eight Presidents in U.S. history, etc. You scale the discussion to the age group. (And hopefully, the lesson is more than just memorizing a list.)

The notion of having a "gatekeeper" in science is ridiculous. Facts are not decided by votes of elitist groups unless we're talking about a church. The IAU should never have taken up this entire issue, as doing so put it in the position of gatekeeper, which was never appropriate and caused a huge mess. They weren't even the inventors of the term dwarf planet, as its use goes back at least to 1991, when the term was proposed by Alan Stern. I disagree to about it being a powerful statement to say there are eight and only eight planets but have no problem with 390 or more dwarf planets. I just don't get why the smaller objects in hydrostatic equilibrium, which in many ways are similar to planets, shouldn't be counted as a lower end of planet, the way brown dwarfs are counted on the lower end of the stellar category. I think eight-year-olds are way more fascinated by the idea of foreign worlds than their specific names and wouldn't mind having a whole lot more of them.

@Raven @Bryan Interesting discussions, but not really germane to the point of the post. The point is: whether or not you want to call these things planets or anything else, let's at least count them correctly!

@David It is my impression from press releases like this one http://www.iau.org/public_press/news/detail/iau0807/ that the IAU is gatekeeping.

One question on the data. Since the primary of 1999 TC36 is binary, shouldn't the size be substantially less than that listed, and therefore probably *not* a DP? Per the 2010 Icarus article, A1 is ≈ 286 km and A2 ≈ 265 km; ≈ 414 km is their "effective system diameter".

If we use science, Pluto is a planet. If we use politics and talk about killing planets, then Pluto is Mickey's dog. If Earth was in Pluto's orbit, it would not be a planet. In fact, since Earth shares its orbit with at least two asteroids, the IAU could deplanetized Earth as well, using the ridiculous 2006 definition of a planet. Soon the IAU will change this absurb defintion when New Horizons reaches Pluto and makes scientists realize the awesome planetness of Pluto.

If we use science, Pluto is a planet. If we use politics and talk about killing planets, then Pluto is Mickey's dog. If Earth was in Pluto's orbit, it would not be a planet. In fact, since Earth shares its orbit with at least two asteroids, the IAU could deplanetize Earth as well, using the ridiculous 2006 definition of a planet. Soon the IAU will change this absurd definition when New Horizons reaches Pluto and makes scientists realize the awesome planetness of Pluto.

@Mike Oh, yeah, the post! Very cool, by the way, and I fully agree - even if I lost sight of what we were talking about. :)

@Anonymous Kids can still memorize the 8 "major" planets at a young age and have interesting discussions as they get older about "other kinds of planets." I know not everyone will have my unhealthy obsession with dwarf planets (business man by day, dreamer of distant worlds by night), but I want everyone to know they're out there. One of the greatest moments of my life was when I realized that the moon wasn't just some crescent-shaped atmospheric phenomenon, but a whole actual PLANET close enough that we could fly to it. I want my kids - and everyone's kids - to have a similar opportunity to behold the wonders our immediate cosmic neighborhood has to offer.

We seem to be caught up in a tangled web of definitions, classifications, categories, jargon etc., all in the name of science; I think not.

Science is to help codify understanding of our universe. Science should be usable by all, not just scientists. We use objective criteria only, when employing scientific principles. Politics, egos, ambition, culture, religion, pride, reputation and the like should be kept in a lock box when using science.

I sure hope that the IAU abandons its ill-conceived notions, confusing rules, and requirements etc., in favor of a neat, clean and polished approached to astronomy and science in general.

It sure is difficult for lay people such as me, to understand all of the perplexing elements that the IAU adopted and seems fit only for an elitist scientific body. It is also egotistical for the IAU to think that all of this is for their benefit, only.

It seems clear, that in a decade or two, the IAU (with a better understanding of the newly discovered bodies in our Solar System and will have to go back and clean up the mess they made.

Now that exo-planets are beginning to be discovered, are we going to argue over another, perhaps more puzzling way, to classify them?

I am sure glad that the IAU did not write our Constitution; if they did, our wonderful country would be swallowed up by a very unscientific black hole.

Obviously categorizing an object as "star", "planet", "dwarf planet" or anything else is only relevant for the big picture. It helps to be able to talk in shortcuts. For individual cases only the actual values and measurements matter. No physical value on Pluto changed one bit in 2006, obviously.

Still, I think it would help if the IAU released what they now apparently feel as their responsibility for dwarf planets. Dwarfs are currently only 'recognized' if the IAU is 100% sure about their 'roundness'. But how can they be sure if even that is not an exact physical parameter (Vesta is a good example where roundness is disputable)? The current list of 5 dwarfs is very misleading.

So the IAU should give up entirely administering the dwarfs and leave it up to the researchers. We will get many databases with many different entries. But so what? As you said very correctly, that's science. Over time many entries will converge.

And we might want to leave it up to someone at Wikipedia to summarize all those databases :-) .

Just noticed that your list doesn't include mass. For binaries, an accurate mass is likely to be found sooner than an accurate diameter (like Pluto, Eris, Haumea, Quaoar, and Orcus ). Dwarf likelihood could also be inferred from the mass in these cases. E.g., what's the theoretical possiblility of something > Yg not being round?

I read this because the new images of Vesta by Dawn and re-reading the Stern/Levison(2000) and Soter(2006) papers reminded me of the 5th anniversary, not of Pluto's demotion, but of the incredible and long overdue PROmotion this first known trans-neptunian and its discoverer received back then. I always felt the whole 'UB313 story in all its detail and strife finally put Tombaugh firmly on the high pedestal he had earned himself long ago doing the mental equivalent of hard labor that none of the v.i.professional astronomers seemed to have time for at the time, regarding how long all those plates were waiting in the archives before they were blink-compared by him. Whether it was their lack of endurance or their energy spent fighting off the latter-minute claims to the observatory from Lowell's heirs - they all missed the Pluto precoveries going back to 1909 or so. Not the IAU or anybody else, but the fact that he did not, and worked hard for it like the proverbial farmboy from Kansas flogged out of college by hailstorm, has put Tombaugh among the great first discoverers of the Solar System: Herschel, Piazzi, LeVerrier, Adams, Tombaugh - visual, mathematical, photographic. (Sorry, no-one yet to add 'CCD' to the list at this level. :-/ But I think 'nearly certainly' at the level shared with purposeful patience and dilligence and Karl Ludwig Hencke... ;-) And then, it took more than 60 and 70 years, respectively, to discover just one more object in the same region as Pluto, and then one of similar size out there. For the main belt asteroids, it took only 3 to 7 years for the first few to follow in the same sense. What's more, some of those great first discoveries were actually quite borderline cases in one way or another; e.g. Uranus and Vesta being naked-eye objects at least occasionally, Pallas and Juno bright and close enough to be stumbled upon in the chase after Ceres, or Neptune being only close to the center of the huge error bars by a good measure of luck and best estimate. Pluto clearly was not. It could not have been found much earlier (maybe ten, fifteen years or so at most for the first precovery plates of the same observatory archive series or quality) or with less effort (unless cracking the jackpot on the first try). No other was found by Tombaugh himself trying for many years until he was satisfied that none could be expected to be found with the means at his disposal. Intentionally or not, this was what the IAU finally confirmed five years ago, removing Tombaugh (and 'nearly certainly' others) from the list of la(te)st discoverer(s) of eccentric footnote oddball planet-a-likes - the gatekeeping of which the discovery intervals of Uranus, Neptune, Pluto, Eris might have suggested five years ago. And yes, I still chuckle thinking of Xena and Gabrielle in this context, and hope that the IAU might dare to decide to orbit Eris (everlasting trouble) at a semi-major axis large enough to let a little Dysnomia (lawlessness) reign over its lists and allow observation and common sense determine what to make of all those rounded rocks out there. ... ... Now, where do I put Vesta?? ;-)

Some pretty amazing data recently (april 2012) on the binary Salacia/Actaea system. Now that the mass (4.5E20 kg) and diameter (900 km) of the primary appear pretty accurately determined it turns out that its density is only around 1.2. This is so low that it might be the case that Salacia has voids inside. If it has then it is not in hydrostatic equilibrium. And if that is the case, then the assumption that a diameter of 800 km guarantees 'roundness' no longer holds. Am I right in thinking that the estimated number of dwarf planets could come down from around 400 to maybe no more than 50 or so?

Dear Mr. Michael Brown, i just wanted to let you know that i honestly think that you didnt kill pluto by discovering Eris, Quaoar, Sedna etc. i just finished a book called "the pluto files" by Neil Degrasse Tyson (whom i personally think IS the REAL pluto killer) and the one who started this pluto scandal (which i like to call "Plutogate")

I think Mr. Tyson is the real pluto killer because he "had no compulsion to include pluto"(the pluto files page 78) when he reopened the rose center in 2000. is he prejudiced against pluto? The Rose Center nor Mr. Tyson will see me there. not worth my time.

i read an article in "Astronomy" magazine about Pluto and S. Alan Stern said that the IAU "embarrassed itself" and also used the words "stupid" and idiotic" to describe the IAU. then Mr. Stern, according to Astronomy "played a mean game of what if: what if you moved pluto into Venus' orbit? what if you moved jupiter into Pluto's orbit?"

Earth has stuff going around it, is it a planet?

to me Mr Brown, you are NOT the pluto killer. You introduced me and other astronomy fans to new planets to love and adore. thank you :)

I don't like the idea which is promoted that Pluto is not a planet. It IS a planet. In fact what the IAU decision amounts to, is saying that Pluto is too small to be a "major planet", but it did not really decide that it wasn't a planet. Why else would they call it a "dwarf planet" if it were not indeed a PLANET. I think every large round body that orbits around the Sun, and especially those that have moons (as Pluto does), are definitely planets.

Dear Mike, I've seen the last update of your list of candidates. I remark that Ceres, one of the 5 IAU-recognized dwarfs, is missing in the list -- as well as Vesta, objet for which it is still unclear if it is in hydrostatic equilibrium and that could be at some point considered as a dwarf planet. Thank you in advance for completing the list with those few objects of the main belt and not limiting it to the TNOs.Best regards,Adrien Coffinet - PhD student at the Observatory of Geneva.

Dear Mike,I suggest that Charon also be added to the list, since the Pluto-Charon system is more like a double-dwarf-planet system than one main orbited by another smaller one. The case of the Earth-Moon system as double-planet system, and hence of the Moon as a (dwarf) planet, could also be discussed. The other smaller "cisneptunian" objects that fit your definitions should also be added to the list, currently limited without any really good reason to the transneptunions objects.Cheers,Adrien

I believe that the dwarf planet definition is fine as-is, it shouldn't be a subcategory of planet. If people REALLY insist, I'd be ok with "major planet" and "dwarf planet" both being considered types of planets, but I'd prefer the current system. Anyway, the IAU needs to get off their asses and declare which known bodies are dwarf planets already. I mean how is Sedna, with all its fame, not listed yet? There are 23 known ones with a median diameter estimate of 600 km or more.

I teach cosmology at Fox Observatory and that's one of the favorite questions, specially from parents. I love to show your list and see the faces :O . I just have one pet peeve about "dwarf" planets; the anglicanism dwarf doesn't translate well in many languages, it sounds offensive sometimes. I know it is way above my pay grade as an amateur astronomer, but I'd love to keep it in Greek notation: planet and planetoid.

It's now been a full decade—a decade full of new dwarf planet discoveries and further confirmation of the dwarf-planet status of previously known bodies—and still we have only five "official" dwarf planets. In the meantime, an entire generation of school children has grown up learning that there are eight "real" planets, and memorizing the names of exactly five dwarf planets. A decade of books have been published including exactly five dwarf planets and completely ignoring the rest. And when a new dwarf planet candidate is discovered and announced, as UZ224 was just a few weeks ago, too many major news outlets still talk about dwarf planets in a way that makes it obvious the only one they've heard of is Pluto.

The solution is for the scientific and lay community alike to simply start referring to all dwarf planets as dwarf planets. OR10, Quaoar, and Sedna are not "probable" dwarf planets—they *are* dwarf planets, just as assuredly as Eris, Makemake, and Haumea are. The same goes for Salacia, MS4, Orcus, Varuna, and a host of others that don't even have names. Creating the dwarf planet category and then abandoning has turned out to be even more irresponsible than just saying everything's a planet would have been, because it separates out the "special five" and then ignores the fact that a major reason for the creation of the category was precisely that there were so many of them!

Even by the most conservative estimates, we have already discovered at least 50 dwarf planets. (See my own list at https://goo.gl/9Z2rJe.) That's ten times more than the five many consider to be the only dwarf planets! And when I think about reasons people haven't embraced the larger dwarf planet community, the answer is simple: It's much easier to have a tidy list of exactly eight main planets and five dwarf planets. With such a small list, kids can not only memorize the eight planets, they can memorize the dwarf planets, too! And in a few more years, when the IAU finally comes around to designating another batch of "official" dwarf planets, students, parents, and teachers will probably argue that they can't, because it would just be too hard to memorize them all. Wouldn't that be ironic?

I know you said not all TNOs need a name, but I think 2002 AW197 should be named Waton. Less than 2 weeks before its discovery Germany was the first country to adopt the Euro which was a major event in 2002, and Waton is kind like the Germanic creation deity since he and his brothers created the world from the body of the giant Ymir.

I use this as an oppotunity to discuss Mike Browns List of the possible dwarf planets.

The first category should be divided in two sections: "dwarf planets according IAU" and "almost certain dwarf planets". It just makes not much sense to discuss any longer whether Pluto Ceres is a dwarf planet, because they are, according to the definition. (The definition is an other topic for discussion elswhere)

As I understand the objects on the list are categorised according to diameter:* > 900(800?) km* 800-600* 600-500* 500-400* 400-200* < 200

In my honest opinion the "probably not", everything smaler than 200 km in diameter, should be excluded. There is no trace of evidence for these objects to be even considered. (Until of course we find an object, that is less than 200 in diameter and perfectly round.)

But there is a need for a category of objects, that were seriously discussed as possible dwarf planets (or even planets), but got rejected based on newer evidence like Vesta.

The table uses diameter to decide which category, but the actual criteria is "roundness" There are objects out there that are over 500 km in diameter but not round enough to be a dwarf planet. So a rocky object or something with an iron core can be big and heavy but not round. Another object is just a flufy snowball, but as ice flows it gets round as we can see in some small moons of the gas planets. So diameter is only roughly connected to roundness.